Since a short time it’s now possible to change the DPI / text size inside a remote desktop session.
When you install the hotfix (link below) the settings wont be grayed out anymore so you can change the DPI.
Here you can read more about the HotFix:
Download the HotFix:
Note that you need SP1 or the HotFix wont work, also a reboot is required for the server.